If you are running Windows PowerShell 5 or older, you can use the CIM Instance method and the Win32_OperatingSystem class. The Win32_OperatingSystem class has a LastBootupTime property which shows the time when the computer was rebooted.
Get-CimInstance -ClassName Win32_OperatingSystem | Select LastBootUpTime Output: PS C:\Users\xxxx> Get-CimInstance -ClassName Win32_OperatingSystem | Select LastBootUpTime LastBootUpTime -------------- 15. 10. 2020 21:47:51